1. Field of the Invention
The present invention relates to an electronic musical instrument and, more particularly, to an electronic musical instrument which can add an in-phase musical effect (e.g., vibrato, tremolo, or wow effect) to tones generated by a plurality of tone generation channels to which the same tone color is assigned.
2. Description of the Prior Art
Conventionally, a technique for adding a vibrato effect to a musical tone generated by each tone generation channel (a series of circuits for generating a musical tone in a specific tone color) by using shift registers and adders is known.
For example, Japanese Patent Publication No. Sho 57-22399 discloses an electronic keyboard instrument which has a plurality of tone generation channels and can add a vibrato effect to each of tones with each phases. In this electronic keyboard instrument, frequency data as a constant proportional to a frequency corresponding to a depressed key (frequency of a tone to be generated) is repetitively accumulated at regular time intervals, and a musical tone waveform memory is accessed at the intervals using the accumulation result as an address signal. The musical tone waveform memory stores amplitude of waveforms of musical tones at sequential sampling points. Therefore, the amplitude of the waveform of a musical tone at the respective sampling points are sequentially read out based on the address signal. In this electronic keyboard instrument, the value of the frequency data is periodically changed to add a vibrato effect. More specifically, since the frequency data is accumulated at regular time intervals, if the value of the frequency data is constant, the rate of increase of the address signal is constant, and a frequency of a musical tone waveform read out from the musical tone waveform memory is also constant. Meanwhile, if the value of the frequency data is periodically changed, the rate of increase of the address signal is also periodically changed, and the frequency of the musical tone waveform read out from the musical tone waveform memory is also periodically changed. Thus, the vibrato effect can be obtained.
In order to periodically change the value of the frequency data, a low-frequency oscillator (to be abbreviated to as an LFO hereinafter) is used. The value of the frequency data is periodically modulated based on an output from the LFO.
The LFO comprises a cyclic shift register having a plurality of storage areas, and an adder. Data stored in the storage areas of the shift register are sequentially shifted in response to clock pulses. Data outputted from the last storage area of the shift register is inputted to the adder, and is added to a predetermined value. The output from the adder is inputted to the first storage area of the shift register. Thus, the shift register and the adder constitute a loop circuit. An output is extracted from an appropriate position of this loop circuit (e.g., from the last storage area), and is used as an output from the LFO. The shift register has the storage areas corresponding in number to the tone generation channels.
The pulse intervals (time slots) of the clock pulses sequentially correspond to the tone generation channels. That is, the time slots correspond to the first channel, the second channel, . . . in turn. The last channel is followed by the first channel.
Therefore, the LFO independently and sequentially outputs output signals corresponding to the respective tone generation channels in a time-divisional manner.
According to the above-mentioned prior art technique, a vibrato effect is added from the point of time when an ON event of a key is detected. More specifically, an output from the LFO for adding the vibrato effect is independently added to each of tone generation channels.
Recent electronic musical instruments can set a plurality of tone colors, and can assign some tone generation channels to each tone color. In such an electronic musical instrument, a musical effect such as a vibrato effect must be added to musical tone in each of tone generation channels by using the LFO.
However, when a vibrato effect is independently added to a plurality of musical tones having the same tone color, which are generated by different channels, a phase shift occurs among the respective musical tones. As a result, the generated tones are unpleasant to the ear. For this reason, when a vibrato effect is added to a plurality of musical tones having the same tone color, all the musical tones having the same tone color must be accumulated, and the musical tones must be generated after data are all accumulated or a plurality of data must be derived from middle areas of a shift register, and must be synthesized, resulting in cumbersome processing. It is very difficult (or impossible) for hardware to realize processing for accumulating musical tones having the same tone color or deriving data from middle areas of the shift register and synthesizing these data.
Moreover, since the shift register must have storage areas corresponding in number to channels, a storage capacity is undesirably increased.